. A characteristic histologic feature observed after treatment of experimental animals with hepatotoxicants, such as acetaminophen or endotoxin, is an increase in the number of macrophages in the liver. These cells are typically observed in the liver prior to histologic evidence of tissue injury. Of particular interest is the regions of the tissue that subsequently exhibit signs of injury. The investigator's laboratory was one of the first to suggest that these cells, which consist of resident Kupffer cells and inflammatory macrophages, contribute to the pathogenesis of liver injury. During the past several years, they have generated data that directly support this concept. Thus, following exposure of animals to hepatotoxicants, they found that liver macrophages are "activated" to release pro-inflammatory cytokines including tumor necrosis factor-a (TNFa) and interleukin-1b, and cytotoxic mediators, in particular, nitric oxide, which has been directly implicated in hepatotoxicity. Furthermore, we demonstrated that blocking the activity of liver macrophages, TNFa, or nitric oxide, abrogates hepatotoxicity. Surprisingly, during the course of our studies, we discovered that hepatic sinusoidal endothelial cells also become "activated" following hepatoxicant exposure. In fact, cells acquire physical and functional properties of inflammatory macrophages and they may also contribute to tissue injury. It is the overall objective of our studies to analyze the role of liver macrophages and endothelial cells and inflammatory mediators released by these cells in the hepatotoxicity of acetaminophen. The investigators hypothesize that acetaminophen treatment of rats markedly increases the responsiveness of parenchymal and non-parenchymal liver cells to pro-inflammatory cytokines released by activated macrophages and endothelial cells. This leads to increased production of cytotoxic mediators that induce tissue injury. To test their hypothesis, they will determine if parenchymal and non-parenchymal liver cells become hyper-responsive to pro-inflammatory cytokines released after acetaminophen treatment of animals, examine mechanisms underlying hyper-responsiveness of parenchymal and non-parenchymal cells to pro-inflammatory cytokines following acetaminophen treatment of rats, and analyze the role of nitric oxide in acetaminophen-induced hepatotoxicity. The results of these studies will provide important information on mechanisms underlying chemical-induced liver injury.